Conventionally, in order to reflow bumps each provided on a plurality of electrodes formed on a front surface of a substrate such as, for instance, solder bumps when producing semiconductor devices and electronic components, the substrate is placed in the interior of a heat treatment chamber and heated from a rear surface thereof so that the bumps are melted and reflowed.
Japanese Laid-open Patent Publication No. 2002-210555 and Japanese Laid-open Patent Publication No. 7-164141 disclose a technique for introducing formic acid or the like into the interior of a heat treatment chamber in order to reduce and remove an oxide film formed on a surface of a bump at the time of reflow processing.
In recent years, further miniaturization and high integration of semiconductor devices and electronic components have been developed, and according to this, a distance between electrodes on a front surface of a substrate, namely, a distance between solder bumps is required to be reduced. There arise problems as follows due to the reduction in the distance between the bumps.
FIG. 7A to FIG. 7C are schematic views for explaining a problem of a conventional reflow processing technique of solder bumps. In FIG. 7A to FIG. 7C, an upper drawing and a lower drawing respectively illustrate a state in the interior of a heat treatment chamber and a state in which solder bumps in a rectangular frame in the upper drawing are enlarged.
First, as illustrated in the upper drawing of FIG. 7A, a semiconductor wafer 110 having solder bumps 112 each provided on a plurality of electrode terminals 111 formed on a front surface thereof is placed on support pins 102 in the interior of a heat treatment chamber 101. Here, a case is illustrated where the solder bump 112 is formed in a so-called overhang shape in which its superior portion (umbrella-shaped portion) 112b is larger than its inferior portion 112a (bottom portion) using, for instance, a plating method.
Next, a reducing gas, which is formic acid in this case, is introduced into the interior of the heat treatment chamber 101, and the interior of the heat treatment chamber 101 is heated at given temperature not lower than a reducing temperature of a surface oxide film (not illustrated) formed on a surface of the solder bump 112 and not higher than a melting temperature of the solder bump 112 by a heater 104 disposed at a lower portion in the interior of the heat treatment chamber 101, namely, at a place at which it faces a rear surface of the semiconductor wafer 110. At this time, the surface oxide film is removed and a surface of the solder bump 112 is in a state of being exposed, as illustrated in the lower drawing of FIG. 7A.
Subsequently, as illustrated in the upper drawing of FIG. 7B, a substrate moving mechanism (not illustrated) drives the support pins 102 in a longitudinal direction so that the support pins 102 are moved downward to be approximated to the heater 104. Under this state, the heater 104 heats the semiconductor wafer 110 from its rear surface at given temperature not lower than the melting temperature of the solder bump 112. Since the solder bump 112 is heated from below in this heat treatment, a timing at which the melting starts is different between the inferior portion 112a and the superior portion 112b of the solder bump 112, and the melting is precedently started from the inferior portion 112a. 
At this time, when a separation distance between neighboring solder bumps 112 is small, the neighboring solder bumps 112 come in contact at the superior portions 112b, as illustrated in the lower drawing of FIG. 7B. This is caused by a lean and fall of the solder bumps 112 due to a thermal vibration or the like since a solder viscosity of the inferior portion 112a is lowered by the above heating yet the superior portion 112b is close to an unmelting state. When the separation distance between the neighboring solder bumps 112 is relatively large, the contact between the neighboring solder bumps 112 does not occur even if the solder bumps 112 are leaned, but, as the separation distance becomes smaller, a probability of contact becomes higher.
The once contacted solder bumps 112 maintain the contacted state even being completely melted by the above heating. Accordingly, there is a problem that a bridge 113 is formed between the contacted solder bumps 112 and a short circuit is occurred, as illustrated in FIG. 7C.